A rheo-optical study of shear rate and optical anisotropy in wormlike micelles solutions

The flow behaviour of wormlike micelles solutions composed of the surfactant cetylpyridinium chloride (CPCl) and counter-ion sodium salicylate (NaSal) at a molar ratio [CPCl]/[NaSal] = 2 in brine [NaCl] = 0.5 M in a cylindrical Couette geometry was examined using homodyne PCS and ellipsometry. Homodyne PCS was used to profile local shear rate and ellipsometry to concurrently profile local optical anisotropy of the fluid. Shear thinning was observed and was correlated to an increase in turbidity and a breakdown in the stress-optic law. A stress plateau observed in mechanical measurements was correlated with the partitioning of the fluid into regions of low shear rate/low turbidity/high birefringence and high shear rate/high turbidity/low birefringence. The partitioning observed was inconsistent with a simple interpretation of the lever rule.